This invention relates generally to the biosynthesis of cholesterol reducing agents. More specifically, the invention relates to the biosynthesis of the cholesterol lowering agent mevinolin by certain microorganisms.
Mevinolin (lovastatin; monacolin K; xcex2,xcex4-dihydroxy-7-[1,2,6,7,8,8a-hexahydro-2,6-dimethyl-8-(2-methyl-butyryloxy)-naphtalen-1-yl]-heptanoic acid xcex4-lactone) is one of the most important known cholesterol lowering agents. Mevinolin, as used herein, includes both the lactone and free hydroxy acid forms.
Its open hydroxy acid form is a potent inhibitor of the 3-hydroxy-3-methyl-glutarylcoenzyme A reductase enzyme, which catalyses the formation of mevalonic acid, an early intermediate of cholesterol biosynthesis. Mevinolin is specifically advantageous because, as a result of its application, biosynthetic intermediates with a toxic steroid skeleton, formed at a later stage of biosynthesis fail to accumulate. Mevinolin increases the number of LDL-receptors at the surface of the cell membrane which remove the LDL cholesterol circulating in the blood, thereby inducing the lowering of blood plasma cholesterol level.
Commonly, the active ingredient is produced via fermentation. GB 2046737 discloses that the active ingredient can be produced by some strains belonging to the Monascus genus e.g. by M. ruber 1005 cultivated between 7 and 40xc2x0 C. As a culture medium the aqueous solution of glucose, peptone, corn steep liquor and ammonium chloride was used. The fermentation was carried out for 10 days in aerobic conditions, and 87 mg mevinolin was obtained from the filtrate of 5 liters broth.
U.S. Pat. No. 4,294,926 discloses the biosynthesis of the mevinolin preferably by the application of microorganisms under the deposited numbers ATCC 20541 or 20542 belonging to the Aspergillus terreus genus on a culture medium containing carbohydrates, e.g., glucose, fructose, maltose, as carbon source, nitrogen sources, e.g., yeast, hydrolyzed yeast, hydrolyzed casein, corn steep liquor; and mineral salts, e.g., calcium carbonate, magnesium sulphate, cobalt, ferro, manganic salts at a temperature of 20-37xc2x0 C. Similar procedures are described in U.S. Pat. Nos. 4,420,491, 4,342,767, 4,319,039 and 4,294,846, where the fermentations are carried out for 3-5 days on media containing 1-6% carbohydrates and 0.2-6% nitrogen sources.
German Patent No. 4,402,591 discloses biosynthesis of mevinolin by microorganisms belonging to the Pleurotus genus, e.g., Pleurotus ostreatus, P. sapidus, P. saca, at 25-35xc2x0 C. during 7-14 days cultivation time on surface or submerge cultures.
Canadian Patent No. 2,129,416 discloses the preparation of mevinolin, or in a particular case, mevastatin, with a microorganism belonging to the Coniothyrium genus, e.g., under the deposited number Coniothyrium fuckelii ATCC 74227 on a culture medium containing 3-15% glucose, 0.54% peptone, 0.5-5% amylase, 0.2-1% ammonium sulphate, 0.01-0.1% magnesium sulphate, 0.05-0.2% antifoaming agent, 0.2-1.5% L-isoleucine, 0.2-1.5% L-aspartic acid in the pH range of 5-6. According to the examples the active ingredient concentration of the broth was within 19-430 mg/liter.
Hungarian Patent No. HU 208,997 discloses the application of the holotype strain Aspergillus obscurus numbered as MV-1, deposited under the number NCAIM(P)F 001189. The fermentation is preferably carried out on a medium containing yeast extract and/or peptone and/or casein as nitrogen source(s) and glucose and/or maltose or sucrose as carbon source(s). The activity of the broth at the end of the laboratory scale cultivation is between 400-850 mg/liter.
The foregoing discussion establishes that the development work in the biosynthesis of mevinolin focused on discovery of new mevinolin-producing microorganisms rather than on the development of the fermentation procedure itself. Several references disclose that fermentations can be carried out on conventional and known media with the application of both surface and solid state cultivations. Batch-like procedures were applied, where the behaviors of the procedures depended on the initial conditions. However, technical limitations, e.g., maintaining the most convenient level of ingredients, optimal dissolved oxygen supply and pH, etc., made it difficult to implement continuous corrective actions to ensure more favourable conditions. A given microorganism during the main fermentation stage, depending on its metabolism, requires different conditions/composition of media in order to obtain an optimal growth and production of the active ingredient. The present inventors concluded from their experiments that in the seed culture and at the beginning of the main fermentation, the quantity of the active biomass is very small and variable. Thus, the yield of the fermentations are relatively low and variable. Yields reached at the end of the fermentations, which depended of course on the strain, did not exceed a mevinolin concentration of 850 mg/liter. The present inventors performed a detailed analysis of the whole fermentation procedure from the seed culture stage throughout the end of the fermentation. It was found that in the seed culture preparation stage, both in the case of the known media and execution processes, the quantity of the biomass is too low. Therefore, during the main fermentation, the metabolism of the microorganism and the culture are not adequate.
It is therefore, one object of the present invention is to improve the efficiency of the mevinolin-producing fermentation procedure by forcing the production ability of the microorganism via changing the conditions and the carrying out of the fermentations.
It is another object of the present invention to provide, in ether or both the seed and main fermentation stage, the most convenient chemical and physiological conditions for the metabolism by the microorganism.
It is a further object of the present invention to provide, in ether or both the seed and main fermentation stage, the most convenient chemical and physiological conditions for the metabolism by the microorganism by maintaining in a steady state condition, the growth rate and then, for an extended time, a maximal product formation rate.
These and other objects of the invention are achieved in one embodiment of the present invention by providing a method for producing mevinolin by microorganism in a fermentation process having a seed culture stage and a main fermentation stage, said method comprising:
a) cultivating a microorganism biomass in said seed culture stage to produce an inoculum;
b) transferring said inoculum into a fermentation medium in said main fermentation stage; and,
c) maintaining steady stage conditions in said main fermentation stage, thereby producing a fermentation broth containing mevinolin.
In a preferred embodiment of the present invention, steady state conditions are maintained in the main fermentation stage by one or more of feeding of organic carbon sources; controlling glucose and/or total reducing sugar content; feeding of organic and/or inorganic nitrogen sources; controlling pH; controlling foam level; controlling the mass of the fermentation broth by withdrawals and feedings; and, controlling the dissolved oxygen level. Preferably, the fermentation process in conducted in a submerged culture of the microorganism and at a temperature in the range of from about 24xc2x0 C. to 30xc2x0 C. In a particularly preferred embodiment of the present invention, the microorganism is an Aspergillus species. In yet other preferred embodiments of the present invention, the organic carbon source is selected from the group consisting of glucose, hydrolyzed starch and vegetable oil; the glucose content is maintained at bellow about 0.2% from the 60th hour of the main fermentation stage; the nitrogen sources are selected from the group consisting of corn steep liquor and ammonium hydroxide; pH is controlled to be within the range of from about 5.2 to about 7.0, preferably from about 5.2 to about 6.2, feeding carbon sources and/or base; foam level is controlled by addition of a material for controlling the foam level, the material preferably being a synthetic material or vegetable oil; and, dissolved oxygen level is controlled preferably by stirring and/or aeration of the fermentation broth. In yet another preferred embodiment of the present invention, the inoculum is transferred from the seed culture stage to the main fermentation stage when the pH of the seed culture stage is increasing after having reached its minimum value.
The inventors have discovered that optimal biosynthesis of mevinolin may be performed by adjusting one or more of certain process parameters, steps and/or variables in ether or both the seed culture and main fermentation stages of the biosynthesis process.
During the seed culture phase, the inventors have found these process parameters, steps and/or variables to include supplying the microorganisms with the necessary medium components in easily assimilable form and in the most convenient concentration and, extending the cultivation time by about 10 to about 25%.
In order to obtain a steady state condition during the main fermentation stage, the inventors have found these process parameters, steps and/or variables to include controlling the glucose and/or the total reducing sugar content, maintaining the carbon sources at a suitable minimum level, feeding organic and/or inorganic nitrogen sources, controlling pH, controlling foam level, controlling the mass of the broth by withdrawals and feeding and controlling the dissolved oxygen level by changing the stirring rate and/or aeration rate.
In order to optimize mevinolin biosynthesis, it is not necessary that each of the above-mentioned process parameters, steps and/or variables for either the seed culture phase or for the main fermentation stage be simultaneously adjusted. However, in a preferred embodiment of the present invention, the biosynthesis of mevinolin will involve each of the above-mentioned process parameters, steps and/or variables. In such a preferred embodiment, an advanced metabolic controlled mevinolin fermentation procedure can be carried out in which a steady state condition, i.e. constant pH, glucose concentration, dissolved oxygen, viscosity, volume, etc., can be reached quickly and which can be maintained for a long time providing a yield highly exceeding the results of the known procedures.
Certain advantages may be realized in the seed culture stage by adjustment of one or more of the above-mentioned process parameters, steps and/or variables in that stage. These advantages include, e.g., reduction of the time requirement to reach the xe2x80x9csteady statexe2x80x9d condition by about 20-30% by increasing the number of growth centres and, growth of the active biomass in a more advantageous morphology form, resulting in more favourable conditions for cultivation of the microorganism. As a result of these advantages and the elongated cultivation time, the concentration of the active biomass is almost doubled.
Certain advantages may be realized in the main fermentation stage by adjustment of one or more of the above-mentioned process parameters, steps and/or variables in that stage. These advantages include, e.g., a faster and less fluctuating growth phase and quick formation of a steady state stage that can be maintained for a long time. These advantages result in a considerably increased activity of the fermentation.
Thus, in one embodiment, the present invention is directed to a fermentation procedure for the manufacture of mevinolin with a strain belonging to the Aspergillus genus in submerged culture at a pH between 5.2 and 7.0, at a temperature within 24 and 30xc2x0 C., on a medium containing assimilable carbon and nitrogen sources, and mineral salts, wherein a metabolic controlled procedure is applied in the main fermentation phase in order to maintain the culture in a xe2x80x9csteady statexe2x80x9d stage. In this embodiment, the total reducing sugar is preferably controlled. In the source of the main fermentation, organic carbon sources, e.g., glucose, hydrolyzed starch and vegetable oil are preferably fed. Preferably, the glucose concentration level is maintained below 0.2% from the 60th hour of the fermentation. In the course of the procedure, nitrogen sources such as corn steep liquor and ammonium hydroxide solution are fed. The pH is preferably maintained in the range of from about 5.2-6.2 by feeding carbon source and/or base, e.g., ammonium hydroxide and/or sodium hydroxide. The foam level the fermenter may also be controlled by feeding vegetable oil, e.g., sunflower oil and/or soya bean oil, and/or synthetic antifoaming agent into the broth. Dissolved oxygen is preferably controlled changing the stirring rate and/or aeration rate. In the course of the fermentation, one or more withdrawals are carried out.
In a preferred embodiment, the main culture medium is inoculated by a seed culture having the following composition:
Culture medium with the above composition is completed with the regularly applied micro- and macroelemental salts, e.g., inorganic salts of sodium, potassium, magnesium and iron. The main culture medium is inoculated with a seed culture of which cultivation time is elongated by 10-25%. At the transferring stage of the seed culture the pH is in the increasing phase after its minimum value.
For the fermentation preferably the Aspergillus obscurus strain, its variant, or its mutant, or more preferably the Aspergillus obscurus n. sp. Mv-1 holotype strain deposited under the code number NCAIM(P)F 001189 is used.
The seed culture is inoculated into the sterile main fermentation medium with elongated cultivation time at the increasing phase of the pH after its minimum value. In the main fermentation stage, a xe2x80x9csteady statexe2x80x9d condition with the maximal active ingredient production rate can be maintained for a long time by feeding of the carbon and nitrogen sources in order to supply the nutrient demand; controlling the glucose concentration to avoid the undesirable thickening of the culture and the exaggerated increase in biomass; controlling the stirring rate and aeration rate according to the oxygen demand; combining foam level control with the carbon source demand an appropriate material for both purposes, e.g., a mixture of a vegetable oil and a synthetic agent; maintaining the pH between the range of from about 5.2-6.2 with the feeding of carbon source, e.g., glucose syrup, or base; and, carrying out one or more withdrawals, when the maximal working volume of the fermenter is achieved, or when a mevinolin concentration economical enough to carry out the downstream processing is reached.
By application of these elements, a versatile controllable fermentation procedure can be obtained, which depending on the life cycle is able to provide well-conditioned constant surroundings for the microorganism.
In accordance with the preferred embodiment described above, the present invention procedures yields exceeding those of known procedures, uses considerably less raw material and energy, reduces the quantity of environmentally polluting waste material relative to the unit mass of active material, and better utilizes the fermenter.